Methods have become known under the term “filtered backprojection” but, as 3D methods, that is to say in conjunction with a matrix-like detector array, do not furnish an image quality which is judged to be adequate in practice. This is because what are known as “cone beam artefacts” occur, because of the conical x-ray beam.
In addition, the disadvantage with this method is that redundant data, such as are produced during spiral scanning with low table advance as a result of multiple irradiation of one and the same voxel, is not used. This results in the radiation dose administered to the object under examination being used only incompletely for imaging.
Furthermore, there are thoughts relating to proceeding, in conjunction with 2D methods for image reconstruction, in such a way that preliminary images are calculated in large numbers by means of “filtered backprojection” from initial data which originates from sections of the focal path which are intrinsically inadequate for image reconstruction, with the preliminary images being reformatted to form a final slice only in a second step. These 2D methods are less useful for detector arrays with a large width, that is to say a great extent in the direction of the system axis. This is because an extremely large number of preliminary images then have to be processed, which is a problem even when high computing power is available.
In all the aforementioned methods, the problem arises that, because of possible data redundancy which occurs during the scanning of the object under examination, image artefacts are produced which have a detrimental influence on the image quality.